This invention relates to a high frequency digital signal receiver for an integrated circuit (IC), and more particularly to a new and improved equalizer receiver having size and power requirements which allow it to be integrated into the IC, preferably as a part of a pad cell circuit of the IC. The new and improved equalizer receiver and its method combine the beneficial effects of gain enhancement and frequency equalization to reliably detect relatively high frequency digital signals while overcoming the adverse affects from initial symbol interference (ISI) of signals communicated over a cable, without requiring a separate equalizer and/or amplifier as added components to the IC.
Modern digital communications systems require the transmission and reception of digital signals at a relatively high rate. The digital signals are high-level and low-level voltages, each of which is a xe2x80x9cbit,xe2x80x9d and the rate of communication of the digital signals is measured in terms of bits per second (bps). The present bps rate of communication internally between a processor and its bus-connected components within modern personal computers is typically in the range of 100-600 Mbps. Such high internal communication frequencies are possible because of relatively short communication paths, tight control over the characteristics of the signals and the communication paths, and the use of coordinated system components which are designed to interact with one another in a specific manner at high communication rates.
Relatively high digital signal communication rates become more problematic, however, when dealing with peripheral equipment connected by cables to the internal processor and bus-connected components. Examples of peripheral equipment connected by cables include disk drives, memories with mechanical components associated with their-media storage, and other computers which are connected by cabling or by a local area network. For cable connections, certain standards have been enacted to enhance the uniformity of signal communication, thereby facilitating the interconnection of various components. One well-known standard associated with cabling is a standard known as small computer system interface (SCSI). The SCSI standard requires that signals delivered have certain voltage levels, exhibit certain waveform characteristics with respect to time and otherwise comply with a variety of other requirements. For local area network connections, such as ethernet, certain protocols and standards have also been enacted which specify the characteristics of the signals and thereby enhance the ability for diverse and unspecified computer equipment to interact with each other over the local area network.
In both cable and local area network connections, difficulties in achieving high frequency communication rates arise which are not present in communication between the processor and its bus-connected elements. The cable itself attenuates the signals, thereby diminishing the signal strength, particularly if the length of the cable is significant. It is not unusual for a SCSI cable to extend a number of feet or meters, and it is typical of that local area network cables extend many tens of feet or meters. Enough attenuation of a digital signal will result in it being mistaken for a digital signal having the opposite value (high or low voltage).
Because the cable itself is a complex impedance having both inductance and capacitance, the signal transmission characteristics of the cable are frequency-dependent. The cable itself filters and blocks the high frequency components of the digital signals but usually passes low frequency signals without difficulty. As a consequence, the relatively rapid transition of a digital signal between the high and low levels is changed by the cable to a more gradual transition. A sharp transition is achieved by passing high frequency components of the signal, but those high frequency components are blocked by the high frequency filtering characteristics of the cable itself. A gradual transition of the signal is undesirable because it affords the possibility that the voltage level of the digital signal will be mistaken for the opposite value as a result of the gradual transition. To avoid the problem of high frequency attenuation, the communication rate must be reduced, which is counter to the evolution of higher communication rates in modern computer systems.
A further significant problem with the high communication rates over a relatively lengthy cable is initial symbol interference (ISI). In simple terms, ISI is in adverse influence on a presently occurring digital signal as a result of the previous digital signals which have been communicated over the cable. The residual affects from previous signals adversely influence the present signal. The previous signals have charged or otherwise influenced the capacitance of the cable, so that a transition of the present digital signal must overcome the previous charge on the cable. For example, if a series of low-voltage level digital signals had been communicated over the cable, the cable is relatively discharged. The occurrence of a high-level signal will necessitate charging the cable. The time taken to charge the cable will diminish the value of the present digital signal until the cable becomes charged to the high voltage level of the present signal. The necessity to charge the cable may result in mistaking the present digital signal for its opposite value. In very high frequency communication systems, more than one of the previous digital signals have residual ISI influences, although the residual affect diminishes with each subsequent digital signal.
Local area network communications systems have solved many of the attenuation, frequency filtering and ISI problems by employing pre-compensation or pre-emphasis driver circuits and equalization receiver circuits connected at both ends of the cables which form the communication media of such local area networks. A pre-compensation or pre-emphasis driver circuit boosts the signal level or amplitude of each transitional digital signal applied on the cable. For example, after a series of low-voltage level digital signals applied to the cable, the first high-voltage level digital signal will be boosted in magnitude by some appropriate margin, for example 30 percent. The boosted magnitude of the transition signal tends to overpower the effects of ISI and attenuation. An equalization receiver circuit connects complex impedance elements to the cable to cause a tuned or peaked frequency response at the desired high frequency communication rate. Equalization overcomes the high frequency filtering characteristics of the cable by providing a peaked or enhanced response at the frequency of the communication rate.
Local area networks also successfully use differential signaling to overcome adverse influences. Differential signaling involves a pair of conductors whose individual signals move in relatively opposite directions with respect to one another. The difference in signal level determines the digital signal value. Noise induced into the cable has a minimal effect, because the noise equally influences the signal levels on both conductors, thereby canceling or rejecting those adverse influences.
The use of pre-compensation, pre-emphasis, equalization and differential signaling circuits in local area networks is acceptable, because it is relatively easy to accommodate these additional separate components to each end of the single connecting cable. However, difficulties arise in the context of a SCSI cable or other internal multiconductor computer cable were it is necessary to connect the SCSI or other cable to the ICs contained within the computer itself. In these situations, to build a pre-compensation or pre-emphasis driver circuit as a part of an IC would consume a large surface of the chip upon which the IC is fabricated. Since the frequency response characteristics of a pre-compensation or pre-emphasis driver circuit which uses operational amplifiers and operational transconductance amplifiers is directly related to the power consumed by these amplifiers, conventional pre-compensation and pre-emphasis driver circuits require large sized components on the IC. The larger components diminish the chip area available for incorporating core functional elements on the IC, thereby diminishing the overall functionality of the IC. Moreover, SCSI standards require much higher signaling voltage levels than are used in modern ICs. The SCSI standard for signals specifies a 5 volt tolerance, but modern ICs operate at approximately 1.8 volts in order to reduce power consumption. Therefore, changing the signal level between 1.8 volts IC level and a 5 volt SCSI level further requires additional components on the IC itself, which further consumes space on the IC and diminishes its overall functional capability. Lastly, the pad cell circuits around the periphery of a modern IC are already occupied by a variety of important signal conditioning circuitry, all of which is intended to shape and otherwise condition the signals delivered from or received at these pads. Indeed, in some modern ICs, the pad cell circuitry is so extensive that the internal space within the IC is substantially diminished for the incorporation of core logic circuitry, which already reduces the overall functionality of the IC.
As a consequence of these and other factors, there exists a substantial problem of incorporating circuitry on an IC itself which will allow high frequency digital signal communication over cables, such as SCSI cables, without the use of numerous, substantially-sized separate components. It is with respect to these and other background considerations that the present invention has evolved.
An important aspect of the present invention is the incorporation in an IC of gain boosting and equalization receiver circuitry in a manner which allows the IC to directly communicate with the cable at relatively high bit rates. Another aspect of the invention relates to incorporating a gain boosting and equalization receiver circuit of a relatively small size in an IC, and preferably in the pad cell circuits of the IC, in a manner which does not consume excessive space, which does not require excessive power consumption, and which will operate effectively at the lower voltage levels which power the IC but which will still respond to the relatively higher signal voltages delivered from the cable in accordance with existing communication standards such as the SCSI standards. Another aspect of the present invention involves the incorporation of a gain boosting and equalization receiver circuit in an IC which is capable of operating at relatively high frequencies but which is also capable of backwards-compatible communication at lower frequencies to enable effective communication with the older, legacy communication equipment which is only capable of lower communication rates. A further aspect of the present invention involves a gain boosting and equalization high frequency receiver circuit having size characteristics capable of its use and incorporation with a multi-conductor cable, such as a SCSI cable, which may contain many separate signal channels, for example 27, without consuming excessive space or power.
To achieve these and other new and improved aspects, the present invention comprises an equalization receiver circuit which responds to two differentially-related digital input signals occurring at a communication rate of a predetermined frequency. A first input device of the equalization receiver responds to one input signal and supplies a first drive signal of a magnitude amplified relative to the one input signal by a factor related to the current conducted by the first input device. A first current source is connected to conduct current through the first input device. A second input device responds to the other input signal and supplies a second drive signal of a magnitude amplified relative to the other input signal by a factor related to the current conducted by the second input device. A second current source is connected to conduct current through the second input device. The first and second current sources are separate from one another. A differential amplifier responds to first and second drive signals to supply an output signal related to the relative difference in magnitude of the first and second drive signals. An equalization circuit is connected between the first and second current sources. The equalization circuit has a frequency dependent impedance characteristic which exhibits a minimum impedance and a maximum coupling of the first and second current sources for the greatest current conductivity and hence, the greatest amplification of the relative magnitudes of the first and second drive signals by the first and second input devices, at the predetermined frequency. The more amplified relative magnitudes of the first and second drive signals cause the differential amplifier to deliver a more amplified or boosted output signal.
A preferred aspect of the equalization receiver circuit includes a diminished responsiveness to the two differentially-related digital input signals occurring at a second predetermined frequency which is different and preferably less than the first predetermined frequency, thereby enabling the equalization receiver to be used with legacy communication equipment. The frequency dependent impedance characteristic of the equalization circuit exhibits a relatively increased impedance and a relatively decreased coupling of the first and second current sources to achieve diminished current conductivity and amplification of the relative magnitudes of the first and second drive signals by the first and second input devices at the second predetermined frequency compared to the current conductivity and amplification at the first predetermined frequency. The diminished amplification of the relative magnitudes of the first and second drive signals at the second predetermined frequency causes the differential amplifier to deliver a diminished output signal at the second predetermined frequency. The responsiveness of the equalization circuit at the first and second predetermined frequencies compensates for the relatively significant high frequency filtering and attenuation characteristics of the cable at the first predetermined higher frequency and allows the digital signals to remain relatively unaffected at the second predetermined lower frequency where the cable exhibits relatively insignificant high frequency filtering and attenuation characteristics.
Other preferred aspect of the equalization receiver relates to its incorporation in an integrated circuit, preferably in a pad cell circuit of the integrated circuit. The cable may be directly connected through intervening connectors to the integrated circuit. Each of the a plurality of communication channels of the cable may be connected to one of the pad cell circuits. Current mirroring devices may be connected to the differential amplifier to conduct current in a mirrored relationship to the output signal from the differential amplifier.
Another preferred aspect of the present invention relates to a method of boosting and equalizing a response to two differentially-related digital input signals occurring at a communication rate of a predetermined frequency over a pair of conductors defining a channel of a multichannel communication cable. The method includes supplying a first drive signal derived from one input signal conducted by one conductor of the pair of conductors of the channel of the cable, amplifying the first drive signal relative to the one input signal by a factor related to a current conducted from a first current source by a first amplifying device, supplying a second drive signal derived from the other input signal conducted by the other conductor of the pair of conductors of the channel of the cable, amplifying the second drive signal relative to the other input signal by a factor related a current conducted from a second current source by a second amplifying device, and coupling the first and second current sources together to achieve a combined current conductivity and the greatest amplification of the relative magnitudes of the first and second drive signals at the predetermined frequency.
Further preferred aspects of the method also involve communicating a response to the two differentially-related digital input signals occurring at a second predetermined frequency which is different from the first predetermined frequency. In this case, the method involves substantially uncoupling the first and second current sources from one another to limit the current conductivity and the amplification of the relative magnitudes of the first and second drive signals at the second predetermined frequency to limit the current conductivity and the amplification compared to that which is available separately from each of the first and second current sources. Other preferred aspects of the method relative to filtering and attenuating the digital signals by communication of the digital signals at the first predetermined frequency over the cable, and communicating the digital signals over the cable without substantial filtering and attenuation at the second predetermined frequency. The steps of the method may also be accomplished by using devices integrated into a complementary metal oxide field effect transistor integrated circuit.